$(this) decorates whatever object this points to with the jQuery functions. The typical use case is for this to reference a DOM element (say, a <div>). Then, writing $(this) allows you to use all the jQuery API functions on that <div>.

If this already refers to a jQuery object - usually a jQuery-decorated DOM object - then calling $(this) will have no effect because it's already decorated.

If in your current context if the this is not a jQuery object, you can make it a jQuery element by wrapping it around $(). When your element already is a result of jQuery expression, the this in that case is already a jQuery object. So in that case they both work similarly